Make this for our Tortilla Casserole (Budin Azteca).
Heat oil in a medium saucepan over medium heat. Add onion and garlic; cook, stirring occasionally, 1 minute. Stir in tomatillos, 1 cup water, and the chile. Bring to a boil. Reduce to a simmer. Cover, and cook, stirring occasionally, until tomatillos have softened, about 15 minutes. Drain, reserving 3/4 cup cooking liquid.Let cool slightly.
Working in batches, puree tomatillo mixture in a food processor with reserved cooking liquid. Add cilantro and salt; pulse to combine.
Salsa can be refrigerated in an airtight container up to 3 days.